The invention relates to a process and an apparatus for maintaining precise regulation of the output voltage or current of a push-pull inverter with large fluctuations of input voltage, whereby the regulation fluctuation formed from the output voltage or current has an actual nominal value with respect to a desired standard value, which regulation fluctuation is decreased by varying the switching times of alternately periodically switched transistors connected to the two primary windings of the push-pull inverter transformer.
Single-ended or push-pull choppers, also called inverters, are known for converting direct voltages of a predetermined value into direct voltages of another value. These choppers use transformers having primary windings, and generally have transistor switches in series with the respective windings for interrupting current flow through the primary. Push-pull choppers, as opposed to single-ended choppers, have various advantages, for example, the production of a symmetrical secondary voltage in the transformer, a small reverse blocking voltage on the disconnected transistors, a small and uniform current flow in the transistors, and a high degree of efficiency.
The value of the output voltage or current of the inverter can be influenced by varying the duration of current flow on the primary side of the transformer. This property can further be used for regulating the output voltage or current. The actual nominal value of the output voltage or current may be compared with an adjustable standard value. The regulation deviation obtained by the comparison is used to control a pulse width modulator, which produces conduction control signals for the transistors. Satisfactory regulation in a push-pull chopper depends on the symmetry of the current and voltages in the two circuits in which the transistors and primary windings or primary winding halves are each connected in series. Symmetry may be broken down by variations of the input voltage of the push-pull inverter output stage. An asymmetrical voltage or current arising over a certain period in one of the circuits generates an average direct current through the primary winding or winding halves, which brings about a progressive saturation of the transformer. With a saturated transformer, only a very small impedance is in series with the transistor, and the transistor is thereby overloaded by high currents.